Abstract
The maximal density achieved by 11 freshwater microalgae (mostly Chlorophyta) with a wide range in cell size, grown under standardized conditions, varied as the -0.79 power of their cell volume. The proximity of this exponent to 0.75 (3/4 rule) and the fact that the relationship was not significatively altered by light limitation $(11 \mu Einst m^-2 s^-1)$ suggest that physiological constraints, and not self-shading as proposed in the past, should be responsible for the existence of the size-dependent maximum density. The results also demonstrate a positive relationship between cell size and maximal biomass that is in agreement with previous field observations.